The adipocyte plays a key role in determining the development of obesity. Fetal adipocyte cellularity and biochemistry may have the most bearing on body composition at maturity. Development of adipose tissue in utero involves events associated with cell division and differentiation. These early developmental changes can be closely followed in simple systems which employ in vitro cell culture procedures. In utero, a more complex system is involved, and many factors act in concert to regulate adipose cell development. To identify those factors in utero which have an impact on adipogenesis, it is necessary to characterize normal adipose cell development and the response of adipose tissue to manipulation of this complex system. The endocrinology of adipogenesis will be characterized prenatally. The developmental aspects of adipogenesis in vivo and in vitro will be monitored using histochemical and biochemical techniques. The first series of experiments will test the hypothesis that an altered adipocyte conversion rate or an abnormal adipocyte metabolism in the fetal animal is responsible for genetic obesity. The use of the genetically obese pig models permits a range of studies in the pre-obese fetal animal that are impossible to do in any other species. The second series of experiments will examine the endocrine requirements for fetal adipocyte proliferation and differentiation in vivo and in vitro. The hypothalamic-pituitary axis will be examined as a source of adipogenic factors in the last series of experiments. These studies will lead to identification of specific factors that cause adipocyte abnormalities in the obese condition.